Wide-band vibration energy harvester with stop

ABSTRACT

A device for harvesting an external source of energy includes an electricity generating device, a flexure, and a first stop. Displacement of the flexure is limited by the first stop. The flexure has a vibration amplitude, wherein the vibration amplitude is amplitude the flexure would have if unconstrained by the first stop. The first stop allows the flexure to oscillate with a vibration amplitude that is higher than displacement of the flexure as limited by the first stop. The electricity generating device generates electrical energy while the first stop allows the flexure to oscillate with the higher vibration amplitude.

RELATED APPLICATIONS AND PRIORITY

This application is a divisional of U.S. patent application Ser. No.12/011,702, filed Jan. 29, 2008 now U.S. Pat. No. 7,839,058,incorporated herein by reference. This application claims priority ofProvisional Patent Application 60/898,160, filed Jan. 29, 2007,incorporated herein by reference.

This application is related to the following commonly assigned patentapplications:

-   “Energy Harvesting for Wireless Sensor Operation and Data    Transmission,” U.S. Pat. No. 7,081,693 to M. Hamel et al., filed    Mar. 5, 2003 (“the '693 patent”).-   “Shaft Mounted Energy Harvesting for Wireless Sensor Operation and    Data Transmission,” U.S. Pat. No. 7,256,505 to S. W. Arms et al.,    filed Jan. 31, 2004 (“the '505 patent”).-   “Slotted Beam Piezoelectric Composite,” U.S. Pat. No. 7,692,365    to D. L. Churchill, filed Nov. 24, 2006, (“the '365 patent”).-   “Energy Harvesting, Wireless Structural Health Monitoring System,”    U.S. Pat. No. 7,719,416 to S. W. Arms et al., filed Sep. 11, 2006    (“the '416 patent”).-   “Sensor Powered Event Logger,” U.S. Pat. No. 7,747,415 to D. L.    Churchill et al., filed Dec. 22, 2006 (“the '415 patent”).-   “Integrated Piezoelectric Composite and Support Circuit,” U.S. Pat.    No. 7,646,135 to D. L. Churchill et al., filed Dec. 22, 2006 (“the    '135 patent”).-   “Heat Stress, Plant Stress and Plant Health Monitor System,” US    Patent Application 2008-0074254 to C. P. Townsend et al., filed Sep.    7, 2007 (“the '254 application”).-   “A Capacitive Discharge Energy Harvesting Converter,” U.S. Pat. No.    7,781,943 to M. J. Hamel & D. L. Churchill, filed Jan. 23, 2008    (“the '943 patent”).

All of the above listed patents and patent applications are incorporatedherein by reference.

BACKGROUND

The vibration energy harvesting beam described in the '365 patentattempts to maximize the strain of bonded piezoelectric patches andmaximize the electrical output by providing a slotted, tapered vibratingbeam that places the piezoelectric patches away from the neutral axis ofthe beam. Such a vibrating beam is especially useful when the ambientvibration level is low and if the vibrating beam may be tuned to beresonant at the predominant frequency present in the instrumentedcomponent, machine, or structure to which it is mounted. Such an energyharvester was tuned to generate electricity to power a wirelesstemperature and humidity sensing node from ambient vibration, asdescribed in the '254 application.

However, in many cases the ambient vibration level may be much higherbut the predominant frequency may be inconsistent or unpredictable. Forexample, aboard helicopters the predominant vibration frequency may bethe rotational rate of the rotor assembly times the number of rotorblades in the assembly. Thus, the structure of the Sikorsky H-60helicopter, which has four rotor blades and has a typical rotationalrates of about 4.3 Hz has a predominant vibration frequency of about16-17 Hz. The G levels have been reported to vary significantly withlocation from about 1 to about 5 G's. Other rotating structures on thishelicopter experience fundamental vibration frequencies that may belower, such as the pitch links or control rods, which vibrate with therotational rate of the rotor assembly of about 4.3 Hz, but which alsocontain higher frequency components. What is needed is an energyharvester design that will generate electricity efficiently under a widerange of vibration amplitudes and frequencies.

SUMMARY

One aspect of the present patent application is a device that includes apiezoelectric flexure and a first stop. The piezoelectric flexuregenerates electricity when the piezoelectric flexure strikes the firststop.

Another aspect of the present patent application is a device, comprisinga bi-stable piezoelectric flexure and a circuit. The circuit includes asolid state voltage dependent switch and an inductor. The bi-stablepiezoelectric flexure includes a first stable position and a secondstable position. The voltage dependent switch is connected between thebi-stable piezoelectric flexure and the inductor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a, 1 b are side views of one embodiment of a wideband energyharvester with a composite cantilever beam that includes a piezoelectricflexure and length-constraining elements located adjacent thepiezoelectric flexure that allow the composite cantilever beam to snapbetween two stable positions;

FIG. 2 a is a top view of the wideband energy harvester shown in FIGS. 1a, 1 b;

FIG. 2 b is a top view of a step in one embodiment of a process forfabricating a wideband energy harvester;

FIG. 2 c is a side view of another step in the embodiment of a processof fabricating a wideband energy harvester;

FIG. 3 is another embodiment of a wideband energy harvester with acomposite cantilever beam that includes a piezoelectric flexure andlength-constraining elements located adjacent the piezoelectric flexurethat allow the composite cantilever beam to snap between two stablepositions;

FIGS. 4 a, 4 b are three dimensional views of another embodiment of awideband energy harvester with a bowl shaped substrate that allow thesubstrate to snap between two stable positions;

FIGS. 5 a, 5 b are side and end views of a step in one embodiment of aprocess for fabricating a bowl shaped wideband energy harvester;

FIGS. 6 a, 6 b are top views of steps in another embodiment of a processfor fabricating a bowl shaped wideband energy harvester;

FIGS. 7 a-7 d are side views illustrating how a bowl shaped widebandenergy harvester may be biased so it can be used with a force providedin only one direction;

FIG. 8 is an embodiment of a compliant vibration harvester that uses atapered flexure element and curved overload constraint;

FIG. 9 a is another embodiment of a compliant vibration harvester withdiscrete end stops to prevent overload and provides a point thatintroduces curvature in the piezoelectric flexure;

FIG. 9 b is another embodiment of a compliant piezoelectric flexure witha mechanical pivot that allows the piezoelectric flexure to be highlycompliant, allowing it to oscillate at a wide range of frequencies;

FIGS. 10 a, 10 b are an embodiment of the compliant vibration harvesterof FIGS. 9 a, 9 b with springs to counter the force of gravity;

FIGS. 11 a, 11 b are embodiments of circuits derived from commonlyassigned U.S. patent application Ser. No. 12/009,945 that take advantageof intrinsic capacitance of a piezoelectric device and that provide thisstorage at the high voltage of the device through a rectifier and avoltage dependent switch to an inductor and capacitor network; and

FIGS. 12 a, 12 b show more detailed embodiments of the circuits of FIGS.11 a, 11 b.

DETAILED DESCRIPTION

As described in the '693 patent:

Numerous sources of ambient energy can be exploited for energyharvesting, including solar, wind, thermoelectric, water/wave/tide,rotation, strain, and vibration. For shipboard monitoring applicationsbelow deck and for monitoring tire pressure and temperature, mechanicalenergy harvesting devices, such as those that harvest strain orvibrational energy are preferred. In Navy applications, strain energywould be available on engine mounts, ship hull sections, and structuralsupport elements. Vibrational energy would be available on dieselturbine engine components, propeller shaft drive elements, and othermachinery and equipment. This energy could be harvested usingelectromagnetic devices (coil with permanent magnet), Weigand effectdevices, and piezoelectric transducer (PZT) materials. Of these, the PZTmaterials hold the most promise.

In one embodiment of the present patent application a flexure element isused that is mechanically bi-stable. Piezoelectric flexure 20 of energyharvester 22 is stable at two extremes of its motion, as shown in FIGS.1 a, 1 b. During the transition in-between these two stable extremespiezoelectric flexure 20 “snaps” suddenly from stable position A tostable position B.

If enough inertial load is provided to mass 24 piezoelectric flexure 20will snap between stable positions A and B at a wide range of ambientload frequencies or vibration frequencies. Energy harvester 22 may beconsidered to be “wideband” since it is capable of efficiently producingelectrical energy at a wide range of ambient vibration frequencies. Therange can be tailored by adjusting mass, stiffness of the piezoelectricflexure and its dimensions. It can even efficiently generate electricitywith a single event provided the single events provide enough force tocause the piezoelectric flexure to snap to the other stable position. Inanother embodiment it can be combined with a tuned cantilever harvesterto provide features of both.

The sudden change in position of piezoelectric flexure 20 from positionA to B occurs because piezoelectric flexure 20 is under compressivepre-loading to create a curvature in piezoelectric flexure 20. In thisembodiment, piezoelectric flexure 20 generates electrical energy whenthis curvature is reversed based on the applied mechanical energy fromvibration or load to the machine or structure to which it is attached.In one embodiment, composite cantilever beam 26 includes piezoelectricflexure 20, length-constraining elements 28 located adjacentpiezoelectric flexure 20, and mass 24, as shown in FIGS. 1 a, 1 b, andin FIG. 2.

In this embodiment piezoelectric flexure 20 is longer than adjacentlength-constraining elements 28. Because they are shorter and mountedbetween the same support structure 29 and mass 24, length-constrainingelements 28 put piezoelectric flexure 20 under compression, causingpiezoelectric flexure 20 to curve. When mass 24 is subjected to asufficient load from either a directly applied force or from anacceleration due to vibration input, piezoelectric flexure 20 moves fromone stable curved position to another. Length-constraining elements 28are momentarily stretched during the transition. Thus, as mass 24 wasdeflected away from stable position A in FIG. 1 a, andlength-constraining element 28 was stretched, the curvature inpiezoelectric flexure 20 rapidly reversed, and piezoelectric flexure 20“snapped” its shape from convex to concave, as shown in FIG. 1 b.

Piezoelectric flexure 20 includes substrate 30 and piezoelectric patches32 a, 32 b mounted to substrate 30. End 34 of composite cantilever beam26 is fixedly mounted to support structure 29 while end 38 of compositecantilever beam 26 includes mass 24. Piezoelectric material called“macro fiber composites” are available from Smart Materials, Inc.,(Sarasota, Fla.), and piezoelectric fiber composites are available fromAdvanced Cerametrics, Inc, (Lambertville, N.J.).

Adjustment of the mechanical compliance of piezoelectric flexure 20 canbe made by changing its stiffness or by changing the amount of mass 24.Stiffness of piezoelectric flexure 20 depends on the material of whichsubstrate 30 is made and its cross sectional area, as well as thecontribution to stiffness from piezoelectric flexure 20.

In one embodiment, if a more compliant composite cantilever beam 26 isused with the same mass, energy harvester 22 can operate reliably inapplications where the vibration amplitude includes lower G levels. Thepresent inventors recognized that substrate 30 and length-constrainingelements 28 both contribute to the stiffness of composite cantileverbeam 26 and that the stiffness of composite cantilever beam 26 can beadjusted to match the expected vibration or loading amplitude. A softermore compliant composite cantilever beam 26 needs less mass to snap tothe other stable position, given the same force. The mass can also beadjusted, with a larger mass delivering more force to compositecantilever beam 26, allowing it to operate at a lower G level.

Support structure 29 moves with vibrating or oscillating component,machine, or structure 40, providing energy to composite cantilever beam26. Mass 24, connected to free end 38 of composite cantilever beam 26 isfree to oscillate when subjected to vibration or movement. When mass 24is subject to inertial loads or a directly applied force, substrate 30suddenly snaps from stable position A to new stable position B, becausethese two positions represent the lowest energy state for substrate 30with mass 24. Substrate 30 may be constructed of hardened steel,titanium, or super elastic nickel-titanium.

Piezoelectric patches 32 a, 32 b bonded to the upper surface 42 andlower surface 44 of substrate 30 respectively are connected by leadwires 46 to energy harvester electronics 47 which receives electricityfrom piezoelectric patches 32 a, 32 b during this sudden snapping event.The electricity can be used to drive light emitting diode 48. The energyproduced by these harvesters can be stored in one or more capacitors orthe energy can be used to charge and re-charge thin film batteries, suchas those available from Infinite Power Solutions (Golden, Colo.). Thebattery may be located within electronics enclosure 49. Once enoughenergy has been stored, smart electronics modules, such as thosedescribed in the commonly assigned '693 patent, allow the load to drawfrom this energy store to perform a task. These tasks may includesampling of sensor data, storage of sensor data, sending data over awireless link to another location, receiving data or instructions fromanother location, and/or storing and forwarding information to anotherlocation.

Piezoelectric patches 32 a, 32 b bonded to either side of substrate 30produce large voltage pulses that may exceed 200 volts each time thesudden shape change snapping event occurs. As described in the '943patent, Capacitive Discharge Energy Harvesting (CDEH) converters areespecially well suited for use with mechanical energy harvestingelements that receive energy from high voltage piezoelectric materials.In one embodiment significant charge is accumulated within thepiezoelectric material itself, improving efficiency. In anotherembodiment, the voltage threshold upon which energy is released from thepiezoelectric and into the energy storage elements of the circuit may beadjusted to take advantage of the voltage provided by the piezoelectricin actual operation.

The present applicants used a CDEH circuit, as described in the '943patent, to efficiently provide the voltage required by LED 48. In oneexperiment, applicants combined the sudden pulse of energy from bondedpiezoelectric patches with a CDEH circuit to light up a blue LED withevery pulse. Preliminary measurements using a digital storageoscilloscope indicated that the energy generated from piezoelectricpatches 32 a, 32 b exceeded 12 microJoules per pulse.

The energy provided by multiple pulses from piezoelectric patches 32 a,32 b and a CDEH circuit can be stored and used to power a wirelesssensing node and a radio frequency (RF) communications module, such asan SG-LINK from MicroStrain, Inc. (Williston, Vt.). In preliminaryexperiments, the present applicants found that approximately 20 secondsof cycling at roughly 2 Hz generated sufficient energy to allow theSG-LINK to sample a 1000 ohm strain gauge and to transmit these dataalong with a unique radio node identification address (RFID).

Substrate 30 and length-constraining elements 28 can also be fabricatedfrom a single sheet of material, such as spring steel, as shown in FIGS.2 b, 2 c. First, slots 52 are formed in material 54 by a process, suchas machining, stamping, chemical etching, or laser cutting. Next, region56 between slots 52 is lengthened by a process such as pressure frompress 58 a on rigid hollowed press surface 58 b as shown in FIG. 2 c, toprovide a bowed shape to region 56. The bowed shape permits two stablepositions of region 56. Next, piezoelectric patches are bonded to bothsurfaces of region 56, a mass is added to one end of material 54, andthe other end is ready to be clamped to the support structure from whichenergy will be harvested.

Another embodiment of a snap action wideband vibration energy harvesteris shown in FIG. 3. Piezoelectric flexure 60 of this embodiment isconstrained at both ends 62 a, 62 b by V-grooves 64 a, 64 b machinedinto end blocks 66 a, 66 b respectively. V-grooves 64 a, 64 b aredesigned to receive edges 68 a, 68 b of long curved piezoelectricflexure 60. Edges 68 a, 68 b are machined as knife edges to remain inplace in V-grooves 64 a, 64 b while maintaining the capability ofpiezoelectric flexure 60 to quickly change from convex to concave andvice versa upon loading of mass 69. Loading of curved piezoelectricflexure 60 is from the force generated by the acceleration of mass 69.

Adjustable length rods 70 a, 70 b have threaded ends 72 a, 72 b, 72 c,72 d that extend through clearance holes 74 a, 74 b, 74 c, 74 d in endblocks 66 a, 66 b. Adjustable length rods 70 a, 70 b can be shortened orlengthened by changing the position of four threaded fasteners 76 a, 76b, 76 c, 76 d at each threaded end 72 a, 72 b, 72 c, 72 d of adjustablelength rods 70 a, 70 b. Shortening of adjustable length rods 70 a, 70 bcompresses piezoelectric flexure 60, causing it to buckle and to havetwo stable positions. As adjustable length rods 70 a, 70 b areshortened, piezoelectric flexure 60 curves more and thereforeexperiences greater strain when snapping between its two stablepositions. The shortening of rods 70 a, 70 b also increases the inertialload required to allow mass 69 to snap piezoelectric flexure 60 from onestable position to the other stable position. Inertial loads applied tomass 69 cause piezoelectric flexure 60 to snap from one to the other ofthe two distinct stable positions.

The inertial load required to snap piezoelectric flexure 60 may beadjusted by changing the stiffness of springs 86 a, 86 b, 86 c, 86 dwhich are positioned between end blocks 66 a, 66 b and threadedfasteners 76 a, 76 b, 76 c, 76 d. Further adjustments may be made bychanging the amount of mass 69 and/or the stiffness of piezoelectricflexure 60. In the embodiment depicted in FIG. 3, end blocks 66 a, isfixed relative to the vibrating or oscillating component, machine, orstructure and vibrate with that structure. End block 66 b is free. Aninertial load from the vibration applied to mass 69 causes piezoelectricflexure 60 to snap from one to the other stable position. An appliedload, such as from a finger or foot, can also be used, and in this caseno mass is needed.

Piezoelectric patches 88 bonded to upper and lower surfaces 90 ofsubstrate 92 provide energy through lead wires 94 to energy harvesterelectronics module 96. Electrical energy provided may be used toilluminate a light emitting diode or may be stored in a battery whichmay be located in a compartment within an enclosure along with theelectronics that may be located in or connected to end block 66 bsimilar to that shown in FIGS. 1 a, 1 b and 2.

In one embodiment, bowl shaped substrate 100 is bi-stable, as shown inFIGS. 4 a, 4 b. Bowl shaped substrate 100 can either have a concave bowlshape, as viewed in FIG. 4 a, or it can snap to a convex bowl shape, asviewed in FIG. 4 b. Substrate 100 is mounted to a support structure atsubstrate end 102 and has mass 104 mounted to free end 106. Mass 104 isattached in central location 107 allowing substrate 102 to snap from onestable bowl shape to another. Substrate 100 may have center region 108cut out, facilitating snapping between its two stable positions.Piezoelectric patches 110 a, 110 b, 110 c 110 d are bonded to upper andlower surfaces of substrate 100, and these patches generate a pulse ofelectricity every time substrate 100 snaps between its stable positions.

Bowl shaped substrate is curved in two planes, as shown by curves 111 a,111 b of FIG. 4 a and curves 111 a′, 111 b′ of FIG. 4 b.

Bowl shaped substrate 100 may be fabricated of a material such as springsteel. Using press 116 that has curvature in two planes, as shown inFIGS. 5 a, 5 b, spring steel substrate 100 is pressed against rigid form118 with press 116 to provide substrate 100 with concave curvature intwo planes: a bowl shape. With this bowl shaped curvature provided,substrate 100 now can snap between two stable positions, as shown inFIGS. 4 a, 4 b.

Bowl shaped substrate 122 can also be fabricated by cutting out slot 124in flat substrate 126, as shown in FIG. 6 a. Ends 128 a, 128 b are thenconnected together to provide substrate 122 with a bowl shape with teardrop shaped slot 124′, as shown in FIG. 6 b. Ends 128 a, 128 b may beconnected with a weld or rivet. Piezoelectric patches 129 are bonded toupper and lower surfaces of substrate 122. With this bowl shapedcurvature, substrate 122 now can snap between two stable positions, asshown in FIGS. 4 a, 4 b.

In many uses, energy may be obtained from the bowl shaped piezoelectricflexure so formed when it snaps in each direction. For example, whenmounted on a vibrating machine, the vibration may equally force bowlshaped piezoelectric flexure from one stable position to the other andback again to the first due to the inertial load created by accelerationof the mass which is affixed to the bowl shaped piezoelectric flexure.However, in some applications, a force is available primarily in onedirection. For example, a force may be provided to piezoelectric flexure130 by a person's foot primarily in a downward direction when the personis walking, as shown in FIGS. 7 a, 7 b. In this embodiment, after bowlshaped piezoelectric flexure 130 snaps toward stable position 2 fromstable position 1, bowl shaped piezoelectric flexure 130 comes incontact with spring 132 located in recessed area 134, as shown in FIGS.7 b-7 c, to restore piezoelectric flexure 130 to its ready positionbetween steps when the force is removed, as shown in FIG. 7 d. Bowlshaped piezoelectric flexure 130 has bowl shaped stable position 2 whenpresence of restoring spring 132 is disregarded.

While electricity is generated when piezoelectric flexure 130 snaps ineither direction, the substantially greater downward force needed toovercome both the tension in bowl shaped piezoelectric flexure 130 andto generate a restoring spring force means that the mechanical energy inboth directions ultimately comes from the stepping action.

An embodiment of an energy harvesting device that has cantilever beam200 well protected from overloads, allows cantilever beam 200 to be verycompliant, as shown in FIG. 8. Tapered cantilever beam 200 may beconstructed of hardened steel, titanium, or super elasticnickel-titanium (Nitinol, Memry Corp). The taper provides a constantstrain field in the area where piezoelectric patches 202 a, 202 b arebonded to the cantilever beam 200, as described in the 115-002application.

Vibration and/or inertial loads applied to mass 204 cause cantileverbeam 200 to move within upper and lower constraints defined by curvedsurfaces 206 a, 206 b of housing 208. Curved surfaces 206 a, 206 b allowcantilever beam 200 to oscillate over a wide range of vibration levelswithout risk of failure due to fatigue of cantilever beam 200 or damageto piezoelectric patches 202 a, 202 b bonded to cantilever beam 200.Thus, cantilever beam 200 can be very compliant and cantilever beam 200will still generate electrical energy without breaking even whenvibration amplitude is high.

Cantilever beam 200 is clamped within housing 208 in area 210 and isfree to oscillate and vibrate from clamped line A-A′ to free end 212.Mass 204 on free end 212 of cantilever beam 200 oscillates due tovibration of the component, machine, or structure to which housing 208is affixed.

Housing 208 also contains energy harvesting electronic module 214 whichis wired to piezoelectric patches 202 a, 202 b and to a battery inbattery compartment 216.

In another embodiment, compliant energy harvesting device 218 providesprotection from overloads and provides electrical generation over a widerange of excitation frequencies, as shown in FIG. 9 a. Discrete stops220 a, 220 b and 222 a, 222 b provide fulcra around which taperedcantilever beam 224 rotates while limiting the strain experienced bytapered cantilever beam 224, preventing overload. Stops 220 a, 220 b and222 a, 222 b also provide higher frequency resonance points as theeffective length of tapered cantilever beam 224 is reduced when itencounters each stop. The reduction in effective length of taperedcantilever beam 224 will be accompanied by an increase in its natural(resonant) frequency as dictated by the following equation for acantilever beam.Wn ²=3EI/l ³Where Wn is the natural frequency of tapered cantilever beam 224, E isits Young's modulus, I is its moment of inertia, and l is its length.

In this embodiment, electrical energy is collected by piezoelectricpatches 226 a, 226 b bonded to the upper and lower surfaces ofcantilever beam 224 each time cantilever beam 224 strikes stops 220 a,220 b and 222 a, 222 b.

In one embodiment, wideband energy harvester may include pivot 230through section A-A′. Pivot 230 may include pinned joint 232, allowingcantilever beam 224 to freely move between stops 220 a, 220 b and 222 a,222 b. Pinned joint 232 can be thinned-down section 234 withincantilever beam 224, as shown in the detail of section A-A′ in FIG. 9 b.Lead wires 238 emanating from piezoelectric patches 226 a, 226 b extendalong neutral axis 236 of cantilever beam 224, cross over pivot 230, andconnect to electronics module 240. Lead wires 238 are coiled in order toprevent fatigue due to cyclic motion in the area of pivot 230.

Pivot 230 introduces an extremely high compliance to rotation ofcantilever beam 224. In this energy harvesting system, cantilever beam224 is unconstrained in all positions, except when it bangs against astop. Under conditions of vibration or cyclic loading, beam 224 willrock or bang between stops 220 a, 220 b and 222 a, 222 b. When beam 224encounters these stops 220 a, 220 b and 222 a, 222 b, strain is createdin piezoelectric 226 a, 226 b which in turn generates energy that isharvested by electronics module 240.

Harvesting energy with energy harvesting device 218 with pivot 230begins when enough vibration amplitude is present to cause themechanically unstable mass 246 to oscillate and thereby cause cantileverbeam 224 to cycle between the two mechanically stable end stop positions220 a, 220 b and 222 a, 222 b. Because pivot 230 is designed to beextremely compliant torsionally, mass 246 will move under conditions oflow frequency vibration as well as higher frequencies. As shown, pivot230 has a very thin section within cantilever beam 224. This thinsection can be machined or formed with a press, allowing pivot 230 toact as a pinned joint that little resists rotation of cantilever beam224.

Compliant energy harvesting device 218 can be optimized for a givenapplication by adjusting the compliance of pivot 230, the length ofcantilever beam 224, the position of stops 220 a, 220 b and 222 a, 222 brelative to pivot 230, and the compliance of stops 220 a, 220 b and 222a, 222 b.

Compliant wideband energy harvesting device 218 can be mounted in anyposition. For example, it can be mounted in a vertical orientationrelative to gravity, so that mass 246 hangs downward like a pendulum,with pivot 230 located above mass 246. In this orientation, side to sidemotion of the component, structure, or machine to which housing 248 isaffixed will cause cantilever beam 224 to encounter stops 220 a, 220 band 222 a, 222 b and generate energy.

Alternatively, pivot 230 and cantilever beam 224 may be located belowmass 246. Cantilever beam 224 will have stable positions when restingagainst stops 220 a, 220 b and 222 a, 222 b. In this case, two verycompliant springs 244 may used to maintain cantilever beam 224 and itsmass 246 in a midline relative to stops 220 a, 220 b and 222 a, 222 bunder conditions of no vibration, as shown in FIGS. 10 a, 10 b.

Cantilever beam 224 can also be mounted in a horizontal orientation, asshown in FIG. 9 a. In this case, a single light spring 244 may be usedto counteract the moment created by the weight of mass 246. One end ofspring 244 would be connected to cantilever beam 224 and the other endto housing 248. Spring 244 would be placed so that cantilever beam 224will remain in a mid position under conditions of no vibration. Whenplaced in a vibrating environment, cantilever beam 224 will move rapidlybetween the stops 220 a, 220 b and 222 a, 222 b, resulting in strain incantilever beam 224 and in piezoelectric patches mounted to cantileverbeam 224, which generates energy which is harvested by the electronicsmodule.

The energy harvesting devices of the present application can be used forradio frequency identification tags for tracking inventoried items,pallets, components, subassemblies, and assemblies. With the energyharvesting devices described herein, consumable batteries would nolonger be needed for operation, and all energy could be derived frommovement or from a direct force input, such as a push button snap actionswitch. The push button switch generates energy by direct application offorce to snap the beam from one curved shape to another curved shape.

The energy harvesting devices can also be used in shoes for children,runners, and bicycle riders to provide electrical energy. For examplethe shoes may include a light that lights up or flashes when subject todirect pressure from walking, or from the changing inertial load ofrunning, thereby making the wearer more visible to vehicles andincreasing the safety of the wearer.

Toys, such as a handheld shaker that lights up when shaken, also couldbe used with the energy harvesting device of the present application.All energy could be derived from mechanical movement, such as shaking.

A wireless switch also could be used with the energy harvesting deviceof the present application that in which pressing the button of theswitch provides a force that causes the bi-stable element to snap,generating enough electrical energy to wirelessly transmit an RFIDsignal. When received by a processor, the processor switches a relaythat controls a light or any other device.

The energy harvesting device of the present application can also bemounted on a fishing lure such that sufficient energy is harvested tolight up an LED when the lure is moved through the water.

The energy harvesting device of the present application can also bemounted on a rotating part, such as a drive shaft, for powering sensorsthat sample and store the operating load of the drive shaft, and thatrecord its loading history.

The energy harvesting device of the present application can also bemounted on a structure or vehicle, such as an airframe, earth movingequipment, a bridge, dam, building, or other civil structure forpowering sensors that sample and store operating strain, and/or loadsand record strain and/or loading history. Networks of such wirelessenergy harvesting nodes may be deployed as appropriate in order to gaininsight and knowledge about the overall behavior of the structure orvehicle.

In each of the above applications a battery can be used for storingenergy harvested by the energy harvesting device, and the batteries canbe automatically recharged without user intervention or maintenance.

As described in commonly assigned U.S. patent application Ser. No.12/009,945 (“the '945 application”), the present applicants designedcircuit 300 a, 300 b that substantially improves energy conversionefficiency, as shown in FIGS. 11 a, 11 b that are derived from thatpatent application. Circuits 300 a, 300 b take advantage of intrinsiccapacitance 302 of piezoelectric device 304 to store charge generatedfrom mechanical strain or vibration, providing this storage at the highvoltage of the piezoelectric device and eliminating loss from charginganother potentially mismatched capacitor. One side of piezoelectricdevice 304 is connected to ground. Diodes provide a positive polarity tothe entire electrical signal generated from the back and forth movementof the piezoelectric device. Once a threshold voltage has been reachedvoltage dependent switch 306 in the circuit rapidly discharges thatstored charge through a rectifier and through a high speed switch toinductor and capacitor network 308 a, 308 b that converts to a lower DCvoltage suitable for use powering electronic circuits. Because theentire charge on intrinsic capacitance 302 of piezoelectric device 304is rapidly discharged no oscillator is needed for this DC-DC conversion.Eliminating the oscillator removes an important source of powerconsumption while maintaining a high efficiency energy transfer.

Unlike previous converter designs, in the present embodiment, whenswitch 306 is off piezoelectric device 304 is not substantially loaded,and is disconnected from almost all sources of loss. Thus, its voltagecan rise quickly to a high value when mechanical energy is applied topiezoelectric device 304. Only when the voltage across piezoelectricdevice 304 has risen to the threshold of voltage dependent switch 306,and voltage dependent switch 306 turns on, is energy first drawn frompiezoelectric device 304 to ultimately charge storage capacitor 310. Abattery can be used in place of or in addition to capacitor 310.Threshold is chosen to be slightly less than the expected open circuitvoltage for expected mechanical excitations. In one embodiment thresholdwas set to 140 volts. In previous designs, such as the embodimentsdescribed in the '693 patent, current was drawn from the piezoelectricdevice as soon as the generated voltage exceeded the two diode forwarddrops of the full wave rectifier plus the voltage from charge alreadystored in the storage capacitor from previous energy conversions. Theseprevious designs wasted energy because they did not allow voltage torise to a high value. By contrast, in the circuit of FIGS. 2 a, 2 b ofthe '945 application, by delaying transfer of charge until the thresholdvoltage is reached, the present circuit design can achieve substantiallyhigher energy conversion efficiency. The threshold voltage is set to beslightly less than the expected open circuit voltage to achieve greatestefficiency.

Energy stored in a capacitance can be described asE=½C V ²where C is the capacitance, and V is the voltage across the capacitance.Because the energy stored depends on the square of the voltage, highvoltage type piezoelectric materials provide substantial advantage.However, the high voltage and high impedance of such materials alsointroduces difficulty in converting to the low voltage and low impedanceneeded by typical electronic circuits. By using intrinsic capacitance302 of piezoelectric device 304 instead of providing a separatecapacitor, as in the '693 patent, the present inventors found a way toretain the high voltage and high impedance through this first stage ofcharge storage, significantly improving energy conversion efficiency.

Piezoelectric device 304 is modeled as generator 320 with intrinsiccapacitance 302 in parallel, as shown in FIGS. 11 a, 11 b. As mechanicalenergy is applied to piezoelectric device 304 on its dependent axis,intrinsic capacitance 302 is charged to a voltage proportional to theapplied mechanical energy. One embodiment, further described hereinbelow, provides that when the voltage on capacitance 302 reaches apreset threshold, switch 306 closes, allowing the charge on capacitance302 to flow into inductor 322. Inductor 322 stores energy in a magneticfield while switch 306 is closed and current is flowing from intrinsiccapacitor 302 in piezoelectric device 304. When intrinsic capacitor 302has discharged to a second threshold voltage, voltage dependent switch306 opens, current through inductor 322 decreases rapidly, and thismagnetic field around inductor 322 collapses. The second thresholdvoltage may be set to provide for nearly complete discharge of intrinsiccapacitor 302. The rapid reduction in current and rapid collapse of themagnetic field when switch 306 opens induces a voltage across inductor322 according to the equationV=L Di/DT

This induced voltage across inductor 322 provides a current throughdiode 324, 324′ charging large storage capacitor 310. This voltageacross storage capacitor 310 is substantially lower than the voltageacross piezoelectric device 304. A correspondingly higher charge isstored on capacitor 310.

The present applicants designed an efficient voltage dependent switchwith very low off state leakage current and a very low on stateresistance to enable operation of this circuit, as shown in FIGS. 12 a,12 b that are derived from the '945 application. Because piezoelectricdevice 304, 304′ voltage dependent switch 306, 306′ and inductor 322 areall in series, leakage current through voltage dependent switch 306,306′ does not detract from the efficiency of the circuit. Leakagecurrent just goes to charge storage capacitor 310.

To operate most efficiently, switch 306, 306′ closes at a firstthreshold when the voltage on intrinsic capacitance 302 is slightly lessthan the expected maximum open circuit voltage piezoelectric device 304,304′ will attain for the mechanical energy input. Switch 306, 306′ lateropens at a second threshold when intrinsic capacitance 302 is nearlydischarged. Switch 306 has been designed to attain a very low resistancequickly when closed to avoid resistive losses. It also has a very highresistance when open, allowing very little leakage current.

The more detailed embodiment of the circuit of FIGS. 11 a, 11 b shown inFIGS. 12 a, 12 b includes voltage dependent switch 306′ that includesDarlington transistors 330 and 340. Each of these transistors needs onlymicro-ampere base currents to turn on, and the Darlington arrangementprovides a very high gain. The two Darlington transistors 330 and 340are arranged in the circuit so that the turning on one causes the otherto also turn on and vice versa. The two Darlington transistors 330, 340remain latched up until intrinsic capacitance 302 of piezoelectricelement 304′ has nearly discharged and the voltage provided fromintrinsic capacitance 302 has declined to close to zero. At that pointDarlington transistors 330 and 340 turn off and reset for the next timecharge is available from piezoelectric device 304′. P. P. Darlingtontransistor 330 has part number FZT705 and NPN Darlington transistor 340has part number FZT605. Both are available from Exodus, Manchester, UK.

Darlington transistor 340 remains off while the voltage across its baseemitter junction 1-3 remains below its 1.2 volt turn on threshold. Thisvoltage is controlled by a voltage divider formed by resistors 342 and344. In practice, any leakage current through Darlington transistor 330from collector to emitter adds to the current through resistor 342 andforms part of this voltage divider. When a threshold of approximately150 volts is provided by piezoelectric device 304′ and applied acrossvoltage dependent switch 306′, the voltage at transistor 340 baseemitter junction, reaches the 1.2 volt turn-on threshold, and transistor340 turns on. The voltage across resistor 346 and across thebase-emitter junction from pins 2-3 of Darlington transistor 330 nowalso equals at least 1.2 volts, and transistor 330 turns on. Thisprovides a high voltage to the base at pin 1 of Darlington transistor340, keeping the transistor on. While the two Darlington transistors330, 340 remain thus latched up, intrinsic capacitance of piezoelectricelement 304′ is nearly completely discharged into inductor 322 throughdiode 360. Voltage dependent switch 306′ continues to conduct until theintrinsic capacitance of piezoelectric element 304′ is nearly completelydischarged.

Since voltage dependent switch 306′ always turns on at the samethreshold voltage, and since the intrinsic capacitance of thepiezoelectric device is also a constant, every closure of switch 306′transfers the same amount of energy, independent of the energy of themechanical event producing it, so long as the energy of the mechanicalevent is sufficient to reach the threshold.

Rather than using a full wave bridge rectifier as in the embodiments ofthe '693 patent, one side of piezoelectric device 304′ is connected toground and shunt diode 365 is used to provide that the entire signalfrom piezoelectric element 304′ and its intrinsic capacitance 302 ispositive. Thus, the peak voltage provided by piezoelectric element 304′is twice the value that would be provided from the same mechanicalexcitation applied to a circuit using a full wave bridge rectifier thatprovides a signal centered at 0 volts.

While this half wave rectifier configuration is desirable forapplications where mechanical energy input is cyclic, a full wave bridgerectifier can be used where mechanical energy input is random infrequency or is of unknown direction. With a full wave rectifier, halfthe voltage is reached but twice as often. Thus, the type of rectifierused determines both the magnitude of the voltage achieved and how oftenthe switch fires.

As described in the '505 patent, and referring to FIGS. 13 and 14 ofthat patent:

As shaft 117 spins, coils 116 mounted on shaft 117 spin as well. As eachcoil moves through the field produced by permanent magnet 118a that coilexperiences a changing magnetic field. The changing field experienced byeach coil induces an emf or an electrical pulse in that coil which canbe converted to DC power using rectifiers 119 . . . .

Thus, pulses of electricity are generated in coils 116 mounted on shaft117 from the motion of shaft 117 and coils 116 through a stationarymagnetic field, and this energy can then be used to power componentsmounted on rotating shaft 117 without any electrical connection to shaft117 . . . . Other than coils, devices such as Weigand wire elements,could be used to generate the emf in place of coils 116.

As described in the '415 patent, and referring to FIGS. 1a and 1b ofthat patent:

The AC magnetic field can also be generated by a magnet moving relativeto coil 82. Such movement may be provided by a vibrating or rotatingbody in the vicinity of coil 82. This vibration could be produced by animpact event.

While the disclosed methods and systems have been shown and described inconnection with illustrated embodiments, various changes may be madetherein without departing from the spirit and scope of the invention asdefined in the appended claims.

The invention claimed is:
 1. A device for harvesting an external sourceof energy, comprising an electricity generating device, a flexure, and afirst stop, wherein displacement of said flexure is limited by saidfirst stop, wherein said flexure has a vibration amplitude, wherein saidvibration amplitude is amplitude said flexure would have ifunconstrained by said first stop, wherein said first stop allows saidflexure to oscillate with a vibration amplitude that is higher thandisplacement of said flexure as limited by said first stop, wherein saidelectricity generating device generates electrical energy while saidfirst stop allows said flexure to oscillate with said higher vibrationamplitude.
 2. A device as recited in claim 1, further comprising ahousing, wherein said flexure is connected to said housing.
 3. A deviceas recited in claim 1, further comprising a second stop, whereindisplacement of said flexure is limited by said second stop.
 4. A deviceas recited in claim 3, wherein said flexure is connected to said housingto vibrate across a center line, wherein said center line has a firstside and a second side, wherein said first stop is on said first sideand said second stop is on said second side.
 5. A device as recited inclaim 4, further comprising a third stop and a fourth stop whereindisplacement of said flexure is limited by said third stop and by saidfourth stop, wherein said third stop is on said first side and saidfourth stop is on said second side.
 6. A device as recited in claim 3,further comprising a pivot, wherein said pivot allows said flexure tomove freely between said first stop and said second stop.
 7. A device asrecited in claim 1, wherein said electricity generating device providesa voltage, further comprising a circuit for reducing voltage provided bysaid electricity generating device.
 8. A device as recited in claim 7,wherein said circuit includes a voltage dependent switch and aninductor, wherein said voltage dependent switch is electricallyconnected between said electricity generating device and said inductor,wherein said voltage dependent switch has a first threshold, whereinsaid voltage dependent switch remains open until voltage applied acrosssaid voltage dependent switch from said electricity generating device tosaid inductor reaches said first threshold.
 9. A device as recited inclaim 8, wherein said electricity generating device, said voltagedependent switch, and said inductor are all electrically connected inseries.
 10. A device as recited in claim 8, wherein said voltagedependent switch has said first threshold and a second threshold,wherein said second threshold is below said first threshold, whereinwhen said voltage applied across said voltage dependent switch reachessaid first threshold said voltage dependent switch closes so charge fromsaid electricity generating device flows through said voltage dependentswitch and through said inductor, wherein when said voltage appliedacross said voltage dependent switch then falls below said secondthreshold said voltage dependent switch reopens.
 11. A device as recitedin claim 8, wherein said voltage dependent switch is a solid statevoltage dependent switch.
 12. A device as recited in claim 1, whereinsaid first stop has a shape matching shape of said flexure when saidflexure is displaced to reach said first stop.
 13. A device as recitedin claim 1, wherein said first stop allows said flexure to oscillateover a wide range of vibration levels.
 14. A device as recited in claim1, wherein said first stop allows said flexure to freely oscillate andvibrate.
 15. A device as recited in claim 1, wherein said first stopreduces effective length of said flexure when said flexure encounterssaid first stop, wherein said first stop provides said flexure with ahigher frequency resonance point than said flexure would have withoutsaid first stop, wherein said first stop allows said energy generatingdevice to convert higher frequency mechanical energy into electricityeach time said flexure strikes said first stop.
 16. A device as recitedin claim 1, wherein said constraining element includes a first stop,wherein displacement of said flexure is limited by said first stop. 17.A device for harvesting an external source of energy, comprising anelectricity generating device, a flexure, a constraining element, and acircuit, wherein said circuit includes a voltage dependent switch and aninductor, wherein said constraining element provides that movement ofsaid flexure is constrained from free vibration, wherein said voltagedependent switch is electrically connected between said electricitygenerating device and said inductor, wherein said voltage dependentswitch has a first threshold, wherein said voltage dependent switchremains open until voltage applied across said voltage dependent switchfrom said electricity generating device to said inductor reaches saidfirst threshold.
 18. A device as recited in claim 17, wherein saidflexure includes a first stable position and a second stable position,wherein said flexure includes a piezoelectric patch mounted to receive amechanical pulse and generate a pulse of electricity when said flexuremoves from said first stable position toward said second stableposition.
 19. A device as recited in claim 18, wherein said mechanicalpulse is a separate individual mechanical pulse.
 20. A device as recitedin claim 17, wherein motion of said flexure is limited by two stops. 21.A device as recited in claim 17, wherein said flexure has a bowed shape.22. A device as recited in claim 17, wherein said flexure has a bowlshape.
 23. A device as recited in claim 17, wherein said flexureincludes a piezoelectric patch and a substrate, wherein saidpiezoelectric patch is mounted on said substrate.
 24. A device asrecited in claim 23, wherein said substrate includes a first side and asecond side, wherein said flexure includes said piezoelectric patchmounted on said first side and a second piezoelectric patch mounted onsaid second side.
 25. A device as recited in claim 23, wherein saidsubstrate includes at least one from the group consisting of steel,titanium, and a nickel-titanium alloy.
 26. A device as recited in claim17, wherein said flexure includes a mass.
 27. A device as recited inclaim 26, wherein said flexure includes a first stable position and asecond stable position, wherein if sufficient force is provided to saidmass said flexure snaps from said first stable position toward saidsecond stable position.
 28. A device as recited in claim 27, whereinsaid force includes an inertial load.
 29. A device as recited in claim27, further comprising a restoring spring, wherein said restoring springis positioned so when said flexure snaps toward said second stableposition said restoring spring acts to restore said flexure toward saidfirst stable position, wherein said flexure has said second stablebowl-shaped position when presence of said restoring spring isdisregarded.
 30. A device as recited in claim 29, wherein said restoringspring is positioned so said restoring spring does not apply a restoringforce on said flexure until said flexure snaps toward said second stablebowl-shaped position.
 31. A device as recited in claim 26, furthercomprising a support structure, wherein said flexure has a first flexureend and a second flexure end, wherein said first flexure end is mountedto said support structure, wherein said mass is located at said secondflexure end.
 32. A device as recited in claim 31, wherein said secondflexure end is unconstrained.
 33. A device as recited in claim 17,further comprising at least one from the group consisting of a storagedevice and an energy using element, wherein said storage device isconnected for storing electricity generated by said electricitygenerating device, wherein said energy using element is connected forusing electricity generated by said electricity generating device.
 34. Adevice as recited in claim 33, wherein said energy using elementincludes at least one from the group consisting of a light, a sensingnode, a wireless communications device and a processor.
 35. A device asrecited in claim 17, wherein said voltage dependent switch has saidfirst threshold and a second threshold, wherein said second threshold isbelow said first threshold, wherein when said voltage applied acrosssaid voltage dependent switch reaches said first threshold said voltagedependent switch closes so charge from said electricity generatingdevice flows through said voltage dependent switch and through saidinductor, wherein when said voltage applied across said voltagedependent switch then falls below said second threshold said voltagedependent switch reopens.
 36. A device as recited in claim 35, whereinsaid voltage dependent switch is a solid state voltage dependent switch.37. A device as recited in claim 17, wherein said flexure includes afirst stable position and a second stable position, wherein said flexurehas a resonance frequency, wherein said electricity generating deviceprovides about the same amount of electrical energy with each movementfrom said first stable position toward said second stable position at arange of frequencies substantially broader than said resonancefrequency.
 38. A device for harvesting an external source of energy,comprising an electricity generating device, a flexure, and aconstraining element, wherein said flexure has an effective lengthwherein said constraining element allows said flexure to oscillate witha higher vibration amplitude than an amplitude said flexure would haveif unconstrained by said constraining element, wherein said flexure hasa first resonance frequency based on said effective length, wherein saidconstraining element reduces effective length of said flexure and allowssaid flexure to have a second resonance frequency that is higher thansaid first resonance frequency so said electricity generating device canprovide electricity at substantially different frequencies.